Appropriateness of introducing earthworms into sustainable agriculture from the perspective of soil carbon emissions

Abstract

Sustainable agriculture, as an effective method for reducing the risk of soil degradation, is often accompanied by increases in soil fauna populations. However, the effects of soil fauna (such as earthworms) bioturbation on soil carbon (C) emissions remain unclear. We conducted a 31-day incubation experiment to measure the effects of adding straw (1 g) and anecic earthworms on the burrowing characteristics and C emissions using two soils collected from the plow layer and plow pan layers. We also explored the effects of the soil C source (soil organic C (SOC) and exogenous organic matter (OM) inputs) on the soil C emissions induced in the presence of earthworms by meta-analysis. The incubation experiment showed that earthworms increased the loss rate of surface straw (49.4–96.8%; p < 0.05) and the soil C emissions (160.5–455.4%; p < 0.01). Earthworms significantly increased the macroporosity (p < 0.001) and macropore size (p = 0.016), but decreased the macropore number in the plow layer soil. While, only the soil macropore size increased significantly (p = 0.025) under earthworm addition in the plow pan soil. Structural equation models indicated that straw directly influenced the soil macropore size (p = 0.022) and C emissions (p = 0.039) in the plow layer soil, but had no effects on the soil macropore characteristics and C emissions (p > 0.05) in the plow pan soil. Earthworms indirectly affected soil C emissions through effects on the soil macropore size (p < 0.05). Meta-analysis indicated that although earthworms increased C emissions by 51.7% and 25.0% without and with OM inputs (p > 0.224), respectively, the earthworm-induced effects on soil C emissions were not related to OM inputs (p = 0.337). Soil C emissions induced by earthworms decreased as the SOC content increased (from 45.1% to 2.6%), but these effects were not significant at any SOC level (p > 0.154). These findings indicate that the effects of anecic earthworms on soil C emissions can be reflected by the earthworm burrowing characteristics and earthworms increased the soil C emissions in the short term (incubation experiment), but their effects on soil C emissions eventually became non-significant due to reduced anecic-earthworm burrowing activities (most burrows are permanent or semi-permanent) and SOC content increase in the long term (meta-analysis). We therefore recommend that earthworm regulation together with OM inputs could be considered in sustainable agriculture.